1. Field of the Invention
The invention relates to an ultrasonic testing method, particularly of an immersed, cylindrical part. More specifically, the invention relates to such a method for detecting internal defects within billets such as titanium billets used in aeronautics.
2. Description of the Related Art
Many part testing systems which operate in the ultrasonic transmission or reflection mode are known.
Ultrasonic part testing may be implemented using a plurality of acoustic transducers elements of fixed focus to detect defects at different depths in the part being tested. Each transducer emits an ultrasonic beam focused at a predetermined depth in the part. The transducers are fitted with focal planes at increasing depths to allow testing the entire volume of the part from the surface to the center.
Ultrasonic part testing also may be carried out using an acoustic probe comprising several electronically focused transducer elements, in which the focusing is implemented by applying different time delays to the signals emitted by each transducer. When obtaining a part's image and using a grid of transducers as the source/receiver, a major difficulty is encountered in that echoes from the reflection at the microstructure of the material are high relative to echoes from any defects such that the former echoes mask the latter. This problem is compounded when the shape of the part is complex and/or the part has a heterogeneous structure, for instance in the case of titanium billets.
It is known to overcome this problem using an appropriate ultrasonic amplification technique with conjugate phase, also called time reversal. According to this technique, following transmission of an unfocused or barely focused ultrasonic beam and reception of the echo returned by the defect to be detected, the returned echo is re-emitted after its time and shape distribution have been reversed in time. An application of this procedure to ultrasonic, cylindrical part testing is described in U.S. Pat. No. 5,513,532. This technique allows optimal focusing on any defect in heterogeneously structured parts such as titanium parts; however, it suffers the major drawback of being time-consuming in the focusing of the ultrasonic beam, thereby substantially increasing the time required to inspect the part as compared to the conventional procedures using electronic focusing.